Field of the Invention The present invention relates to a sensor to detect NOx concentration in an exhaust gas from combustion furnace and automobile engine, etc.
From the view point of the protection of an environment on the earth, reduction of NOx, especially NO, is required and for the achievement of this requirement, the development of a NOx sensor, which can be applicable to the feed back control of combustion state and NOx removal system or to monitoring oxidation and reduction catalyst of NO, is thought to be essential. A solid electrolyte sensor which detects NOx concentration by the change of electromotive force corresponding to the change of NOx concentration and a semiconductor sensor utilizing the change of the resistance of organic or inorganic semiconductor depending on NOx concentration have been reported as NOx sensors.
For example, the Japanese Patent Laid-Open Publication Nos. Sho 61-184450 and Hei 4-297862 disclose a solid electrolyte sensor using AgI and NASICON as a solid electrolyte and metal nitrate as an electrode. These sensors show relatively good sensitivity for NOx in a temperature range from 100.about.300.degree. C . However, the melting point of AgI used as a solid electrolyte and metal nitrate used as an electrode are low and it was very difficult to use the sensor in a condition exposed to a temperature above 500.degree. C .
Zirconia is used in oxygen sensors as a thermal stable electrolyte at a high temperature. In a NOx sensor using zirconia, it is reported that a good performance is obtained by using mixed salts of Ba(NO.sub.3 ).sub.2 or Ba(NO.sub.3 ).sub.2 with other salts as an electrode. (The 18th Chemical Sensor Meeting, vol.10, p.73.about.76, 1993). However, a long range stability is an issue due to the deliquescence property of Ba(NO.sub.3).sub.2. A sensor using SnO.sub.2, which is stable at a relatively high temperature as an electrode and zirconia as an electrolyte is reported, however, the detectable gas is methane, but not NOx.
A sensor using organic semiconductor phthalocyanine has been disclosed (The Japanese Patent Laid-Open Publication No. Sho 62-95456), however, the working temperature is limited by the decomposition temperature of phthalocyanine and it is impossible to mount the sensor directly in an exhaust gas at a high temperature. On the other hand, a relatively stable sensor at a high temperature using an oxide semiconductor has been disclosed. (The Japanese Patent Laid-Open Publication No. Hei 1-150849). However, there were issues that the sensitivity is lowered or null above 500.degree. C., where the gas absorption on the surface of the semiconductor does not occur easily, since the semiconductor sensor detects the concentration of the gas through the change of the resistance caused by the absorption of the gas on the surface of the semiconductor.